1. Field of the Invention
The present invention relates to a fuel cell that directly uses pure hydrogen, methanol, ethanol, diethyl ether, or reformed hydrogen from methanol or fossil fuels as a fuel, and air or oxygen as an oxidizing reagent, and particularly to materials and a membrane and electrode assembly used in a solid polymer fuel cell, and a fuel cell.
2. Description of the Related Art
In these years, a fuel cell, which is can be utilized as a power source for next generation, is actively studied, and a solid electrolyte, which is an element thereof as a proton conductive material, is also actively studied.
In general, as a proton conductive material, a per fluorocarbon sulfuric acid film as represented by Nation (registered trademark) is used. However, it has not sufficient proton conductivity yet. But, increase in an amount of sulfuric acid group in the polymer structure for the purpose of enhancing the proton conductivity results in decrease in mechanical strength and volatilization in an aqueous solvent. Further, it has such problem that it softens under high temperature conditions (100° C. or more) to result in decrease in the proton conductivity, which makes use of Nafion® difficult at high temperature regions (100-140° C. or more). In addition, there also remains such problems that a monomer to be used is relatively expensive and the complex production method pushes up the production cost.
There are many examples of developing a solid electrolyte using a polymer raw material with a high rigidity. On the other hand, in these years, a solid electrolyte, which uses a resin material with a high solvent resistance among polymer raw materials, has been studied. For example, there are disclosures about a solid electrolyte mainly employing suffocated polyether ether ketene, suffocated polysulfide or suffocated polyether ketene in JP-A-6-49202, JP-A-6-93114, JP-A-8-20716, JP-A-9-245818 and JP-A-10-21943. However, in all of these solid electrolytes, since a sulfuric acid group directly bonds to an aromatic ring in the polymer main chain, there is such problem that the sulfuric acid group is gradually detached due to a high operation temperature to result in lowering in battery performance. Further, since the sulfuric acid group bonds directly to the polymer main chain, distance between the main chain as a hydrophobic site and the sulfuric acid group as a hydrophilic site is short to lead to absorption of water molecules beyond necessity, thereby resulting in such problem that a low introduction amount of the sulfuric acid group makes the compound soluble in an aqueous solvent.
In Japanese Patent No. 3607862, a solid electrolyte is manufactured by bonding a sulfuric acid group, via an alkyl group, to an aromatic ring in the polymer main chain synthesized by polycondensation. The method aims to enhance the proton conductivity and mechanical strength by bonding a sulfuric acid group to the main chain via a ethylene group and thus separating the hydrophobic site of the main chain from the hydrophilic site of a side chain having the sulfuric acid group with a relatively large distance. However, as it now stands in actual, since the reaction efficiency is very low, only a small amount of sulfuric acid groups are introduced and the hydrophilic site of the side chain having the sulfuric acid group is not sufficiently separated from the hydrophobic site of the main chain.